Brushless motors, which are also called commutator less motors, have electric contacts in place of mechanical contacts such as a commutator and brushes. One example of such brushless motors is disclosed in Japanese Patent Laid-open Publication No. 2001-275325 for “Electric Power Steering Apparatus”. The brushless motor disclosed in the No. 2001-275325 publication is an inner-rotor type brushless D.C. motor, which includes a cylindrical outer stator having a plurality of armature windings provided thereon and an inner rotor rotatably disposed in the outer stator and having a plurality of permanent magnets secured to the circumference thereof.
The electric power steering apparatus for a motor vehicle, which is equipped with the brushless motor, is constructed to not only transmit steering torque, applied manually by a human vehicle operator or driver to the steering wheel (hereinafter also referred to as “drive-applied steering torque”), to a pinion shaft of a rack-and-pinion mechanism but also transmit steering assist torque, generated by the brushless motor in response to the driver-applied steering torque, so that steerable road wheels of the vehicle are steered via the rack-and-pinion mechanism. When the steering torque applied to the steering wheel (i.e., driver-applied steering torque) is relatively small, the steerable road wheels are steered in accordance with the driver-applied steering torque with no steering assist torque given from the brushless motor. When, on the other hand, the steering torque applied to the steering wheel is greater than a predetermined value, the steerable road wheels are steered by a combination of the driver-applied steering torque and steering assist torque.
Because the brushless motors generally present a smaller moment of inertia than the brushed motors and no brush-caused loss, they give rise to much less electrical losses. Electric power steering apparatus equipped with such a brushless motor can afford a good steering feel as the driver quickly turns the steering wheel in the opposite direction. Further, the brushless motors, which cause smaller friction torque, achieve advantageous benefits, such as better returning of the steering wheel.
However, the brushless motors have the following problems. Normally, when the armature windings are not being energized, cogging torque (magnetic attractive/repulsive force) acts between the magnetic poles of the outer stator and inner rotor. In the brushless motors having salient-pole windings, there would arise relatively great cogging torque due to a smaller number of magnetic poles, and thus torque variations due to the relatively great cogging torque could become a significant problem. Therefore, it is preferable to minimize the cogging torque. For example, when a motor vehicle provided with an electric power steering apparatus is traveling straight forward, the steering angle of the steering wheel is relatively small. During that time, the steerable road wheels are steered at relatively small angles, in which case only the tire treads of the steerable road wheels deform slightly. Consequently, a road surface reaction force (i.e., frictional resistance between the road surface and the tires) is relatively small, so that only relatively small steering torque has to be applied by the driver to the steering wheel and thus no steering assist torque is necessary.
Namely, where steering wheel may be maneuvered by the driver through small angles around its neutral position and thus the steerable road wheels may be steered with the driver-applied steering torque alone, the steering feel can be effectively enhanced by allowing the driver to constantly maneuver the steering wheel with substantially the same steering torque.
However, when the brushless motor is not being driven because only small steering torque is necessary, not only the steerable road wheels are steered in accordance with the driver-applied steering torque but also the inner rotor of the brushless motor is rotated in accordance with the driver-applied steering torque. Therefore, if undesired cogging torque of the brushless motor is great, torque fluctuations may be produced on the steering wheel, which may undesirably impair the steering feel during high-speed straight forward travel of the vehicle. For this reason, reducing the cogging of the brushless motor can be an important condition or factor for an enhanced steering feel.
To reduce the undesired cogging torque of the brushless motor, it may be effective to slant teeth-shaped magnetic paths where the armature windings of the outer stator are provided, or slant the magnetic poles of the permanent magnets provided on the inner rotor. However, slanting the teeth of the outer stator would complicate the structure of the motor and also require the windings to be slanted with respect to the rotational axis of the rotor. Thus, the conventional winding machine can not be used, and various other manufacturing difficulties would be encountered.
The brushless motor having slanted magnetic paths of the permanent magnets is known, for example, from Japanese Patent Laid-open Publication No. SHO-59-44957 for “Electric Machine Having Energization Control Means”. In the electric machine disclosed in the No. SHO-59-44957 publication, a plurality of permanent magnets are provided on the rotor in such a manner that boundaries between the magnetic poles of the circumferentially-adjoining permanent magnets are slanted with respect to the rotational axis of the rotor. As mentioned in the publication, in the case where a plurality of permanent magnets are positioned on the rotor in a circular arrangement or configuration, the radius of curvature of the inner peripheral surface (i.e., surface adhered to the rotor) of each of the permanent magnets must satisfy a very severe tolerance, e.g. for preventing an undesired runout of the rotor. Further, it is very difficult to uniformly magnetize the slanted magnetic poles from end to end, so that slanting the magnetic pole boundaries with respect to the rotational axis of the rotor is not necessarily an easy task.
For the aforementioned reasons, there are increasing demands for a technique capable of readily minimizing the cogging torque of the brushless motor, as well as an electric power steering apparatus capable of achieving a highly improved steering feel.